Field of the Invention
The present invention relates to a method of and a scrubber for removing pollutant compounds from a gas stream. More particularly, the present invention relates to dry circulating fluidized bed (CFB) scrubbers, used for removing pollutants, such as SO2, HCl and HF, from a stream of exhaust gas.
Description of Related Art
Dry CFB scrubbers are well-known systems for removing pollutants, especially, acid gases, from a pollutant laden gas stream, such as exhaust gas emanating from a combustion boiler. Dry CFB scrubbers generally comprise a gas channel for introducing the gas stream to a lower portion of the scrubber, an inlet chamber at the lower portion of the scrubber for directing the gas stream upwards, a constriction section arranged above the inlet chamber for accelerating the gas, a reaction chamber arranged above the constriction section, means for introducing at least one reagent to the reaction chamber for converting pollutant compounds in the gas stream to reaction products, a discharge channel for discharging gas and particles including the reaction products from the reaction chamber, a particle separator for separating particles including the reaction products from the gas, and a return channel for recycling a portion of the separated particles from the particle separator to the reaction chamber.
The reagent used in dry CFB scrubbers, especially, when removing SO2, SO3, HCl and HF from exhaust gas, is usually an alkaline material, such as hydrated lime Ca(OH)2, CaO, CaCO3, and NaHCO3. Especially, when removing pollutants like mercury, dioxins or furans, existing in low concentrations in some flue gases, other reagents, such as powdered activated carbon, lignite coke or bentonite, can also be introduced into the scrubber system.
The reagent can be injected into different locations, for example, in the combustion process, in the exhaust gas duct upstream of the scrubber, directly into the reaction chamber of the scrubber or into the return channel. The pollutant compounds in the gas stream react with the reagent material to form reaction products, generally solid salts, which are removed from the gas in the particle separator, usually a fabric filter.
Water is usually also injected into the reaction chamber to control temperature and humidity therein, for example, to enhance the reaction of SO2 with Ca(OH)2. In dry scrubbing systems, however, unlike in wet scrubbers, the gas stream that is being treated is not saturated with moisture. To the contrary, only the amount of moisture that can be evaporated in the exhaust gas without condensing is added.
The reaction chamber of a dry CFB scrubber comprises a bed of particulate material, consisting mainly of fly ash and reagent particles, fluidized by a vertical stream of pollutant laden gas. The reaction between the reagent and the pollutant compounds takes place mostly on the surface of the bed particles, mainly in the reaction chamber. In order to maintain the bed in the reaction chamber, usually, a portion of the material collected by the particle separator is recycled back to the reaction chamber.
In a commonly used construction, shown, for example, in the patent document International Publication No. WO 2005/030368, recycled material is introduced from a bottom hopper of the particle separator through a sloping recycling channel and a feeding nozzle to a side of an expanding, downstream portion of the vertical constriction section arranged above the inlet chamber. This solution has the disadvantage that because the feeding nozzle is at a relatively high vertical level in the scrubber, the total height of the scrubber system becomes relatively large.
International Publication No. WO 2006/032288 discloses another construction in which the height of the scrubber system is decreased by connecting the recycling channel by a feeding nozzle to a side wall of the inlet chamber, upstream of the constriction section. This solution has the disadvantage that because the velocity of the gas stream in the inlet chamber may be relatively low, especially near the location of the feeding nozzle, a too large portion of the recycled particles tends to sink to the bottom of the inlet chamber, especially at low loads. This effect can be minimized by recycling gas to the bottom of the inlet chamber, which, however, makes the system complicated and increases the costs.
Chinese Patent Document Nos. CN 201760230 and CN 101402019 show a dry fluidized bed desulfurization reactor comprising a sloped channel for conveying absorbent particles to an absorbent distributor at a central zone of an inlet chamber, at the vertical axis of a constriction section comprising multiple circumferentially arranged venturi nozzles. Even with these solutions, the recycled particles are introduced to the inlet chamber at a zone in which the gas velocity is relatively slow and the entrainment of particles and their distribution to the reaction chamber may be less than optimal.
An object of the present invention is to provide a method of and a scrubber for removing pollutant compounds from a gas stream in which at least some of the problems of the prior art mentioned above are minimized.